A PayPal billing agreement is a master subscription allowing to clear all the generated invoices for current active services or new.
Also, upon developping your application you may generate invoices to order or renew services and domains.
The issue is that the invoices would by default be paid manually which defeat the purpose of automation in some cases.
In order to achieve a full automation, you would be able to set a Paypal Billing Agreement directly from your Manager.
After the billing agreement is being set, all invoices generated from your account would be taken care of automatically.
Therefore, if you were to place an order from the website or even from the API it would be instantly provisionned for you.